Ferromagnetism

Ferromagnetism is the basic mechanism by which certain materials (such as iron) form permanent magnets, or are attracted to magnets. 

Bar_magnet

A common bar magnet. image: wikipedia

In physics, several different types of magnetism are distinguished. Ferromagnetism (including ferrimagnetism) is the strongest type.  It is the only one that typically creates forces strong enough to be felt, and is responsible for the common phenomena of magnetism in magnets encountered in everyday life. 

Substances respond weakly to magnetic fields with three other types of magnetism:  paramagnetism, diamagnetism, and antiferromagnetism, but the forces are usually so weak that they can only be detected by sensitive instruments in a laboratory. 

A-1_horseshoe-magnet-red-silver-iron-filings-AHD

A horseshoe style magnet

An everyday example of ferromagnetism is a refrigerator magnet used to hold notes on a refrigerator door. The attraction between a magnet and ferromagnetic material is "the quality of magnetism first apparent to the ancient world, and to us today".


Permanent magnets (materials that can be magnetized by an external magnetic field and remain magnetized after the external field is removed) are either ferromagnetic or ferrimagnetic, as are the materials that are noticeably attracted to them. Only a few substances are ferromagnetic. 

The common ones are iron, nickel, cobalt and most of their alloys, some compounds of rare earth metals, and a few naturally occurring minerals, including some varieties of lodestone (magnetite is considered ferrimagnetic, rather than ferromagnetic).

Ferromagnetism is very important in industry and modern technology, and is the basis for many electrical and electromechanical devices such as electromagnets, electric motors, , speakers, generators, transformers, and magnetic storage , and hard disks.


Where does magnetism come from?

One of the fundamental properties of electrons (besides that they  carry charge) is that they have magnetic dipole moments, i.e., they behave like a tiny magnets. These dipole moments comes from the more fundamental property of electrons known as quantum mechanical spin

Due to its quantum nature, the spins of the electrons can be in one of only two states; with the magnetic field either pointing "up" or "down" (for any choice of up and down). 

The spins of the electrons in atoms is the main source of ferromagnetism, although there is also a contribution from the orbital angular momentum of the electron about the nucleus. 

When these magnetic dipoles in a piece of matter are aligned, (point in the same direction) their individually tiny magnetic fields add together to create a much larger macroscopic field.

However, materials made of atoms with filled electron shells have a total dipole moment of zero, because the electrons all exist in pairs with opposite spin, every electron's magnetic moment is cancelled by the opposite moment of the second electron in the pair. 

Only atoms with partially filled shells (i.e., unpaired spins) can have a net magnetic moment, so ferromagnetism only occurs in materials with partially filled shells. Because of Hund's rules, the first few electrons in a shell tend to have the same spin, thereby increasing the total dipole moment.

These unpaired dipoles (often called simply "spins" even though they also generally include angular momentum) tend to align in parallel to an external magnetic field, an effect called paramagnetism. Ferromagnetism involves an additional phenomenon, however: in a few substances the dipoles tend to align spontaneously, giving rise to a spontaneous magnetization, even when there is no applied field.


Source: adapted from Ferromagnetism. (2017, February 20). In Wikipedia, The Free Encyclopedia. Retrieved 23:32, March 5, 2017, from https://en.wikipedia.org/w/index.php?title=Ferromagnetism&oldid=766494482